chore: bump version to 1.2.0

- Update version in main.go, build-release.sh, and install.sh
- Update RPM spec file version
- Prepare for v1.2.0 release

Author: Aryamanraj

packaging/rpm/go-sol-sign.spec

@@ -1,5 +1,5 @@
 Name:           go-sol-sign
-Version:        1.0.0
+Version:        1.2.0
 Release:        1%{?dist}
 Summary:        Command-line tool for signing messages with Solana keypairs